This invention relates generally to working and heat treating super alloys and specifically to the manufacture of thin metallic sheet of small diameter bar and wire and other thin section geometries from nickel-base alloys containing relatively high amounts of aluminum and/or other reactive metals. Reactive metals in addition to aluminum are defined as those metals having an equal or greater tendency and propensity to form oxides or nitrides at elevated temperatures than titanium. More specifically, this invention relates to the manufacture of alloy sheet and round products which are to be subsequently brazed or used as a weld filler metal.
It has been discovered by those making sheet and round products from alloys containing significant amounts of reactive elements by the usual process of alternately cold rolling, drawing, or other reduction methods the sheet or round product and then annealing it in an atmosphere with a low partial pressure of oxygen and/or nitrogen, that even with very low partial pressures of oxygen and/or nitrogen, a thin film or skin of very stable oxides and/or nitride will form on the surface during annealing at normal annealing temperatures.
Indeed, the alloys are often designed so that oxides form and exposures of the alloy in low partial pressures of oxygen are utilized to preferentially form these stable compounds. See, for example, U.S. Pat. Nos. 4,312,682; 4,460,542; and 4,439,248.
Unfortunately, the film or skin inhibits the ability of others who wish to braze components made from the sheet to other metallic articles. The film is also undesirable when wire is used as weld filler. Normal brazing practice dictates that surfaces to be brazed be clean and free of oxides, nitrides and foreign substances. Even so, state of the art braze alloys are capable of fluxing small amounts of the less stable oxides such as chromium, iron, and nickel oxides which may form on alloy surfaces.
However, aluminum oxides and reactive metal oxides flux poorly or not at all. In fact, these types of oxides are frequently used in "stop-off" mixtures to prevent braze alloy from wetting undesirable surfaces. Self fluxing brazing alloys do not meet the challenge of removing the oxide film on super alloys with high reactive metal contents.
Typical solutions to the problem of oxides on the surface of conventional alloys are generally concerned with their removal, e.g. by pickling or abrasive cleaning. The most common pickling solution for high alloy nickel, iron and cobalt base alloys is a nitric-hydrofluoric acid solution. This type of solution has been found to be ineffective in completely removing the oxides and films being discussed. Other pickling solutions, if developed, would require a second pickling set up, acid disposal system and excessive cost. In addition, the thinner gage sheets and smaller diameter round products present significant handling problems when they are pickled. U.S. Pat. No. 4,566,939 illustrates one prior approach to solving some of these problems.
Abrasive cleaning would be effective if equipment for handling thin sheet or small diameter round products were developed, but costs would be high. However, abrasive cleaning of thicker cross section intermediate products just prior to the final manufacturing sequence of the final product by the process of this invention is considered beneficial.
Thermodynamic equilibrium data such as that presented in U.S. Pat. No. 4,439,248 and reproduced in FIG. 1 would suggest that to eliminate the formation of aluminum oxides one would anneal or heat treat the product at a combination of high temperatures and very low partial pressures of oxygen corresponding to the upper left corner of the schematic labeled "C" and "essentially free of Al.sub.2 O.sub.3." Similar reasoning would apply to the relationship with oxides of other reactive metals. One method of achieving the high temperatures and low partial pressure of oxygen is to heat treat in an ultra-high vacuum system. Unfortunately, these systems are expensive, not readily available, and do not provide the flexibility required to rapidly cool coils of product.